was Re: [Collins] Best Receivers; now 51S1 critics.

[Gerald]

On Thu, 2005-08-11 at 06:28 -0700, C Eus wrote:
> I have to agree with this one. What is so bad about the 51S1 that it garners such a negative criticism? 
> 
> I think about a year or so ago, there was some ham who made WAS or some other contesting award using some small BA transmitter and a 51S1 receiver. I think this unit is fine for SWL or basic ham purposes. 
> 
> Cal, N6KYR/8
> 
>  
> 
> 
> snip>>>>
> I don't understand what caused the dislike of the 51S-1 receiver, I have never seen one that I did not like. I use one in the shack that I can switch in and out of line, which I do often when DX'ing. The results have been nothing less than outstanding. The 51S-1 will hold it's own or do better alongside any of my other receivers / transceivers.
> 
> Best 73
> 
> Merle W1GZS

My dislike was taught. One day in the early spring of 1964 while on TDY
to Dallas working on the Collins 821A-1, Lloyd Winter, my group head,
the guy who signed my time cards, caught me in the lab drooling over a
51S-1. I explained that I was shopping for a receiver. He said that he
had been in charge of the design of the 51S-1 and the latest 75S-3B and
the 3B was the superior receiver. That I should get the 3B and a crystal
pack for general coverage. So I did and never have spent any time with
the 51S-1. He wasn't a ham but did SWL. He wanted to find an ARR-15 made
by Collins. A receiver he'd also been in on the design. I modified the
75S-3B with a little switch and a single crystal socket mounted on top
the tuning rack front support so I could select the internal crystal
board or that crystal socket that is accessible by raising the receiver
lid less than two inches. Changing crystals in the crystal board soon
got old and without the proper puller (it was decades before I got a
crystal pack) crystals soon developed new shapes from the needle nose
pliers.

I think the biggest problem with most receiver designs boils down to
mixers, mixer noise, mixer strong signal handling, and RF selectivity.
There are three mixers in the 51S-1 at least on some bands to contribute
noise and overload. There are just two in the 75S. The choice of mixer
tube can have a huge effect on how much the mixers limit the dynamic
range of the receiver. In the 75S, the mixers or simple triode or
pentode with cathode injection. Are some of the mixers in the 51S made
of pentagrid tubes like 6BA7? There are noisier mixers, but not much.

For most of the first couple years I owned the 75S-3B, it was out of the
case turned up on its side so I could probe and modify. Working 2m with
a converter (which had to be optimized a great deal itself) I found the
dynamic range of the combination a problem when I pointed my array south
(by then my job had moved to Richardson Texas permanently and I lived in
Allen Texas, 11 miles north of the Collins plant) and W5WXV had his 600
watts out to a bigger array in Garland Texas pointed north. With just
him on the band on 144.085 (before the CW subband was created) I found
him some 18 or 20 times in the first 200 KHz. Exploring the setup, I
found that I could add attenuation between the nuvistor convertor and
the receiver to help, but the problem was overload in the second mixer.
And exploring the stage by stage noise contributions (besides finding a
hum modulated noise from the unshielded plastic cased mechanical filter)
I found the second mixer contributed a great deal of the receiver's
noise. I changed the second mixer from the pentode of a 6EA8 to an
Amperex, 6688 or 7788 (I don't remember which) that had ten times the Gm
and so a much lower noise contribution. Then I moved the second mixer
grid from the top of the IF coil to the unused tap (normally grounded in
the transmitter to make the bottom end of the IF coil available for
mixer neutralization). That was a little more signal reduction than most
effective for HF operation, but it cut the signals from W5WXV down to 3.
One at 144.085 where he was supposed to be showing full scale and the
other two just at the noise level. A great improvement, and yes the
noise figure test set showed the system NF was as good as I'd ever seen.

I certainly heard decently on 2m, far better than I was heard,
especially through Oscar 2.

Could I have done that to a 51S-1? Would I have?

And with a scope on the (added) IF output of that 75S-3B I grew to
dislike the mechanical filters of the age that rang so much that they
filled in the gaps between line noise pulses coming at the rate of 360
per second. Those mechanical filters converted single lightning into
extended crashes.

I mentioned that to the receiver support engineer in Cedar Rapids. He
said, "I wondered why those using Collins receivers gave up with static
got a little strong, that I just noticed as clicks in my HQ-129X."

I found that my first receiver building project, a converter attached to
a Q-5er worked much better during static. And eventually I added the S-
line dual time constant AGC and interconnects for receiver control so I
could connect it or the 75S-3B to my transmitter and in my weekly sked
with my dad (now SK) on 75 many times I went to the Q-5er for solid copy
while he had great problems with his 75S-2. Later I went to a Tentec
Corsair II and found it had more selectable bandwidth than the Q-5er but
also handled static well. Which leads me to not be a great fan of the
transient response of the 60s vintage Collins mechanical filter.

That makes me happy with the 51J-3/R388 in my rack, though some day I'll
probably want to turn it into grocery money while I do my SWL and
hamming with my recently acquired brace of FT-857D. If not rained out
this weekend, the Corsair II may make a swapmeet and hamfest with a
price tag attached. Meanwhile the S-line sits neglected with leaky
original black beauties still in place.

When the receiver IF selectivity comes from IF transformers it is
possible to sacrifice a bit of IF gain but to gain bandwidth by stagger
tuning as prescribed in the R-390 manual.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer